AND THE THEORY OF LIGHT. 67 



All that 1 expected to effect by this method of operating 

 was to produce some tint of green, blue, or purple; and 

 although these colours were produced, and my theory 

 thereby confirmed, still I did not succeed in discovering 

 any definite law, for shades, extremely like each other, are 

 produced by different proportions of black and white on 

 the rings, as well as by different velocities of the machine. 

 Nor is this to be wondered at when we reflect on the infi- 

 nite variety of greens and other colours which is to be 

 seen in nature. This is indeed an object of research, 

 enough of itself to employ the labour of a whole life. 



loo. Although I am unwilling to enter on the discus- 

 sion of diversity of shade at present, I may mention that 

 it requires fewer revolutions per second to produce purple 

 than to produce blue, and fewer to produce blue than 

 green. But it must not be supposed that a colour depends 

 on the number of revolutions of the machine, for every 

 colour is seen on the same disc at the same time. 



The coloured diagrams annexed [plates /., //., ///.) will 

 serve to give some idea of the colours ; but the difl[iculty 

 of producing a representation of such colours as are seen 

 on thin plates, soap bubbles, mother of pearl, &c., will be 

 easily understood. 



Excentric Motion. 



loi. Although by operating with discs having definite 

 ratios of black and white, as in the above examples, shades 

 of various colours were produced, still there was not so 

 great a variety of colour as I desired. Beautiful, there- 

 fore, as these colours were, they did not satisfy me. I 

 imagined that if I could by any possibility make one 

 disc revolve over another I might greatly modify the co- 

 lour. In order to effect this I tried several plans without 

 success. At last it occurred to me that if an excentric 

 motion were given to those figures, or discs, which I 



